The present invention relates to satellite communication systems, and more particularly to satellite communication systems having multiple spot beams that serve as communication links between multiple coupled gateways and a plurality of subscriber terminals, wherein the satellite communication systems can provide virtual gateway redundancy without having to reserve platform resources.
Telecommunications and data communications are interconnected over heterogeneous networks via gateways. Gateways can be located in various locations geographically separated from each other. A gateway is a communication device that interconnects networks at higher layers than routers and converts a communication protocol to another communication protocol. Examples of telecommunication protocols are Frame Relay, ISDN; HDLC, X.25, T1/E1, T3/E3, ATM, SONET/SDH, etc. TCP/IP is a well-known data communication protocol, which is used for Internet communication. Gateways in this description mean generally gateways, routers, switched communication equipment, access nodes, hubs, ground stations, satellite earth stations that translate communication signals between subscriber terminals and services servers via a satellite. Examples of services servers are multimedia servers, email servers, voice mail servers, video-on-demand servers, etc. Terrestrial gateways may comprise a signaling interface to a telephone network such as the Public Switched Telephone Network (PSTN), the Mobile Network through a switching center, and/or an Internet Protocol (IP) network.
Many contemporary service providers are national or multinational in nature. Therefore, they provide services that may span several geographical areas and require several gateways. Service providers sometimes even cooperate with each other to expand their networks. For that, their gateways must be interconnected. In order to guarantee a committed capacity and service up-time, service providers must resort to setting redundant (or backup) gateways, which become active in case primary gateways fail. This create issues regarding the number of idle gateways as well as the inefficient utilization of the existing resources. This lack of full utilization is expensive, especially considering the cost of waste physical storage space and network utilization.
An example of monitoring whether or not a gateway in the interconnect network is active and available is to exchange “keepalive” messages across the connection. If the keepalive messages are not received within a specified interval, the interface protocol connection is dropped, and the gateway will be removed. Gateways in the normal operation may use certain routing algorithm concepts to keep update the Internet Protocol (IP) address of the next-hop gateway in specified intervals. For example, the distance vector routing (also referred as Bellman-Ford Algorithm) algorithm and the link-state routing algorithm are known to one of skill in the art. RIP is a very simple distance-vector based routing protocol. BGP is another distance-vector based routing protocol. A distance vector protocol has each gateway transmit (Destination Address, cost) pairs to that gateway's neighbors. There are two types of RIP formats documented in RFC 1058 and RFC 2453. In the link-state routing, each gateway is responsible for meeting its neighbor and learning their names. Each gateway constructs a packet known as a link-state packet (LSP), which contains a list of the names of and cost to each of its neighbor. The LSP is transmitted to all the other gateways, and each gateway stores the most recently generated LSP from each other. Each gateway, armed with the information in the LSPs, computes routes to each destination. The Dijkstra Algorithm may be used for computing of routes.
Large network service providers such as AT&T and Sprint have routing information for all their network addresses. Their gateways exchange information using Exterior Gateway Protocols, with Border Gateway Protocol version 4 (BGP-4) being the standard and documented in RFC 1771. Gateways use TCP connection to exchange information with neighboring gateways. When a gateway is not available, each end of the connection withdraws all routes going through the now-unavailable neighbor. It is understood that routing protocols are evolving, and they are likely to be obsolete by other RFCs and/or newer versions.
The vast majority of subscribers in urban or suburban areas are served by either hybrid fiber coaxial, cable, or ADSL networks. Both cable and ADSL rely on physical wires to provide network access. The capital expenditure depends on the geographic distance between subscribers and access nodes. The infrastructure cost is shared by all subscribers residing in the area. When the subscriber density is low, such as in the rural or remote areas, the wired infrastructures are too costly to be deployed. An alternative solution is routing information signals to the destined subscriber terminals via satellite. A transmitter at the gateway uplinks the information signals to the satellite, which frequency converts them and forwards them to the destined geographical area through its multiple spot beams.
The satellite is conceptually similar to a base station in a cellular communications network, where the base station is located at a very high altitude above the earth. A geostationary (GEO) satellite is in orbit about 36,000 km above the equator, and its revolution around the Earth is synchronized with the Earth's rotation. Therefore, the GEO satellite appears stationary, i.e., fixed on the Earth's surface.
Like a cellular infrastructure, a satellite network can divide the covered geography (footprint) into many smaller footprints using multi-beam antennas. Gateways in the footprint of one spot beam can communicate with subscriber terminals located in footprints of other spot beams. The term spot beam refers to a directional radiation pattern provided by a satellite antenna in which the area of geographical coverage is constrained to a footprint having a line of sight to the satellite. The spot beams can carry two-way communications, sent in packets at specific time intervals and allotted frequencies. And all wireless technologies for cellular communications such as CDMA, FDMA and TDMA technologies and the combination thereof can also be applied to the satellite communication. Similar to cellular communication networks that employ frequency reuse to maximize bandwidth efficiency, a satellite communication system has the additional advantage of employing orthogonal polarization to further increase available bandwidth.
A spot-beam satellite system may comprise a plurality of orthogonal time or frequency slots (defined as color patterns), a plurality of frequency re-use patterns that can be regular in structures, where a uniformly distributed capacity is required, and a plurality of spot beams.
In a spot-beam satellite system, it is desirable to have gateway redundancy to significantly lessen or eliminate any shortfall against the system's committed capacity during temporary gateway outage periods. The outage could be caused by natural disasters or routine maintenance. In a system with a large number of gateways and user service beams, this can mean adding significant spacecraft platform resources to enable a backup gateway to take over the beams of another gateway.
It is also desirable to have the capability of a phased gateway build-out during the system's early stages without sacrificing coverage over the full service area.